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Propagation of Laguerre-Gaussian Beam in Turbulent Medium

Author: XuJianCai
Tutor: ZhangYiXin
School: Jiangnan University
Course: Optical Engineering
Keywords: Atmospheric turbulence Laguerre–Gaussian beam photon state aberration corrected photon communication
CLC: TN929.11
Type: Master's thesis
Year: 2010
Downloads: 151
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Abstract


Optical communication between satellite and ground is the aim which human has been pursing, for its excellent features. However, it is evitable that optical beam will be disturbed by atmospheric turbulence caused by heat transmission because the communication channel is atmospheric turbulence. Therefore, how to select a suitable optical source and study the effects of atmospheric turbulence on the optical beam is critical. Laguerre-Gaussian beam as a special beam with orbital angular momentum has a wide range of application in many fields. Especially, in theory it carries orbital angular momentum which is infinite. This offers the possibility of realizing arbitrary base-N digit per photon. As a result, it is make great sense to make a study of the effect of atmospheric turbulence on Laguerre–Gaussian beam.In this paper, a optical propagation model with Laguerre-Gaussian beam as the optical resource is established. Firstly, based on the orthonomarlity of Laguerre-Gaussian beam , mode analysis and Rytov approximation, we study the effect of atmospheric turbulence on the Laguerre-Gaussian photon. Meanwhile, we briefly analyze atmospheric turbulence induced beam spread. Secondly,with the conception of wave function in quantum mechanics,the detecting probability of slant propagation of low order of Laguerre-Gaussian single-photon in atmospheric turbulence has been studied as well as the detecting probability after tilt aberration corrected. Thirdly, based on my tutor’s original research ,the effects of some low order atmospheric aberration on orbital angular momentum is numerically calculated and the analysis is offered.The results are show as blow:1 The quality factor of Laguerre-Gaussian beam is a key factor for mode weight distribution. The mode weight of the initial mode is decline as the quality factor becomes large. Moreover, parameter P0 influences the initial mode weight greater at the receiving plane than L0 . The total mode weight scattered to L±ΔL is almost the same. With the increasing of atmospheric turbulence, the quality factor of the beam goes up at the end, and the quality factor is linear with respect to atmospheric turbulence.2 when the atmospheric turbulence is constant, the radius of the detector and the photon number which sender sends to the receiver affects the detecting probability greatly. Under the control of media strong atmospheric turbulence, if we want to obtain the photon detecting probability up to 0.3 when the detector’radius is 0.2 meters, we must send 66680 pulses with the same information. The results also show that the detecting probability of uplink is larger than downlink. The situation of the detecting probability is improved after tilt aberration, and the result is much better under the strong atmospheric than the result under weak atmospheric turbulence.3 The tilt induced cross-talk is the largest among the four low-order atmospheric turbulence which are tilt aberration, astigmatism, coma and defocus. Turbulent coma aberration is second, astigmatism is third and defocus has no effect on orbital angular momentum. The former three atmospheric turbulence have similar effects on orbital angular momentum cross-talk. That is to say, the larger the value of parameter P ,the stronger the Cn2,the farer the propagation distance is ,the stronger the cross-talk is, the stronger the crosstalk is.

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